1. Field of the Invention
The present invention relates generally to automatic flight control systems for aircraft and more specifically to prediction of the commencement of altitude acquisition and the control of the aircraft flight path during actual altitude capture in such a way as to maintain accelerations normal to the flight path at or below a predetermined value.
2. Description of the Prior Art
Most modern commercial transport aircraft, general aviation aircraft and military aircraft are equipped with an automatic flight control system. These automatic flight control systems generally provide the human pilot with the capability of altering the flight path of the aircraft to achieve and maintain a desired flight reference such as Mach number, airspeed, vertical speed, glide slope and the like.
In addition, most automatic flight control systems include an altitude capture subsystem whereby a desired altitude may be preselected by the human pilot while the aircraft is at an entirely different altitude and upon achieving predetermined conditions automatically capturing that altitude. During the climb (or descent) to the preselected altitude, the aircraft's air data computer continuously supplies the altitude capture submode parameters, altitude error, the difference between the preselected altitude and the actual altitude hereinafter referred to as h.sub.e, and the actual vertical speed or altitude rate, hereinafter referred to as h.sub.a. At a predetermined combination of h.sub.e and h.sub.a, the capture of the preselected altitude commences. Generally representative of such a prior art altitude capture submode is that of Miller U.S. Pat. No. 3,240,446 entitled "Preselected Altitude Control System for Aircraft", issued Mar. 15, 1966 also assigned to the present assignee. In the Miller system, altitude capture commences when the term (h.sub.e -Kh.sub.a) reaches a null value and by maintaining (h.sub.e -Kh.sub.a) near zero, an asymptotic flight path to the preselected altitude results. The value of the constant K defines the time constant or duration of the capture maneuver and is generally chosen to provide a timely altitude capture while keeping the aircraft's acceleration normal to the flight path, the g effect, at an acceptable comfortable value. For small values of K, unacceptable acceleration levels may be experienced when the aircraft is at relatively high altitude rates at capture initiation. Conversely, for large values of K, an unacceptable long time-to-altitude capture period occurs with lower altitude rates at capture initiation.
Another prior art altitude capture submode is defined in the present assignee's pending application, Flannigan et al, Ser. No. 197,735, now U.S. Pat. No. 4,377,848 filed Oct. 16, 1980 and entitled "Altitude Capture Mode for Aircraft Automatic Flight Control System", wherein the value of K is proportional to the actual altitude rate of the aircraft at capture initiation thereby commanding a circular arc flight path for the altitude capture maneuver. While this scheme permits the commanding of a predetermined constant acceleration normal to the flight path, it results in altitude capture initiation at very large altitude errors for high initial altitude rates and is susceptible to overshooting the desired altitude as the altitude rate, and hence the value of K, is reduced toward null. In addition, the large values of K commensurate with higher initial altitude rates tend to make the system sensitive to atmospheric turbulence, the effect of which on altitude rate measurements by the air data computer is well known.
In both of the aforementioned altitude capture schemes, a fixed path in space relative to the atmosphere is commanded. The time rate of change of the commanded path represents the acceleration of the aircraft relative to the flight path. In Miller, the commanded path results in an asymptotic approach to the preselected altitude and, hence, a varying normal acceleration. In Flannigan, the commanded path is circular, and hence, represents a constant acceleration throughout the altitude capture.
A significant shortcoming of either scheme is that the resultant normal acceleration on the aircraft is not necessarily that which was commanded at the inception of, or during, the capture maneuver. For example, if the altitude rate of the aircraft is increasing just prior to commencement of the capture maneuver due to an autopilot command to capture a commanded speed slower than the actual speed, the actual flight path of the aircraft will initially differ significantly from the commanded flight path. The resultant correction toward the commanded flight path can cause normal accelerations of an unacceptable level.
The present invention overcomes all of the aforementioned shortcomings. It provides a prediction or anticipation of an incipient altitude capture such that the actual flight path of the aircraft corresponds with the commanded flight path at the commencement of the capture manuever. Hence, unacceptable normal accelerations due to aircraft path correction is eliminated. Furthermore, the present invention alters the determination of the value of K such that the commanded capture results in either an asymptotic capture, a circular capture, or a combination of both depending upon existing conditions.